Carbon black is used as conductive additive to polymers, thermoplastics and rubbers. Polymer/carbon black composites usually show a very typical percolation behavior. The electrical resistivity of the composite follows a curve as shown in FIG. 1.
As illustrated in FIG. 1, the addition of carbon black to a polymer composition is, up to a certain level, without any effect on the DC resistivity, then suddenly the resistivity drops to a low level and evolves further only very slowly, which is well-known phenomenon in the art. The influence of the type of carbon black chosen as a filler is typically only on the concentration needed to achieve the percolation effect, while the resulting resistivity of the polymer/carbon black blend remains at a constant level, regardless of the type of carbon black added as a filler. In some applications, a conductive polymer is, however, not desirable, and the filler should only be used as an antistatic modifier to nonconductive polymers.
Carbon black is also commonly employed as a filler in elastomer (rubber) blends. Such Rubber blends have a great importance in, e.g., the tire industry. Since different rubbers have different types of responses to stress, blending of selected rubbers has been practiced to meet the need of the contradictory set of properties, i.e. yielding rubber products with the desired properties. Blending also improves the processability of rubbers and may overall reduce costs of production.
Rubbers are hardly used in their pure form. They are commonly mixed with reinforcing fillers, non-reinforcing fillers, plasticizers, process aids, antioxidants and vulcanization ingredients to provide the required physical properties and to bring about an optimum level of vulcanization. In contrast, thermoplastics are mixed with few ingredients such as fillers, stabilizers and process aids and are processed at a temperature above their melting point or glass transition temperature.
In fact, carbon black is one of the most important active fillers used in the rubber industry for improving the mechanical and dynamic properties of rubbers. The behavior of these fillers in rubber matrices is very different, mainly because of the difference in their surface characteristics. The surface characteristics of the filler have an important contribution towards the wetting behavior, interaction with the rubber matrix, reagglomeration in the matrix, etc.
One problem commonly encountered with carbon black fillers is that the surface energy of conventional carbon black is normally higher than that of various elastomers like Styrene-Butadiene rubber (SBR), Butadiene rubber (BR), and Ethylene-Propylene-Diene rubber (EPDM). With a large surface energy difference between filler and rubber, the filler-filler interaction increases, which in turn has a negative influence on the stability of the dispersion state attained during mixing. Reducing the surface energy and chemistry to the range of various rubbers may aid in compatibilizing these fillers.
In order to modify the surface energy and properties of the carbon black particles, attempts have been made to coat the carbon black particles with a polymer layer. The creation of a polymer film on the surface results in an altered contact resistance and contact capacity. In other words, polymer/carbon black blends employing polymer coated carbon black show an increased DC resistivity compared to polymers or blends with non-modified carbon black fillers because the electrons will have to tunnel through the additional surface polymer layer of the particle to interact with the surrounding filler particles or polymers.
Surface modification of carbon black by polymerization is generally known in the art. Polymerization of carbon black by conventional polymerization reactions (dissolving the monomer in a suitable solvent, contacting the monomer solution and possible additives with the carbon black particles and subsequent evaporation of the solvent (mostly by heating) to form a polymer layer on the surface of the particles) has been described in the art. For example, depositing epoxy or phenol resins onto the surface of fullerenic soot containing carbon black in the presence of solvents is described in Japanese Published Unexamined Application No. 1996-291295 (JP 08 291295 A) to Tokai Carbon KK.
Conventional polymerization has, however, a number of disadvantages, such as the requirement to apply heat to achieve the immobilization of the layer on the surface, and the unwanted presence of residual solvents or of other additives used to accomplish the polymerization in the final product.
Plasma polymerization has emerged as a surface modification technique for metals, polymers and powders. Plasma polymerization is different from the conventional polymerization processes. The polymer formed from plasma polymerization and conventional polymerization generally differ widely in chemical composition, as well as chemical and physical properties, even if the same monomers are used for polymerization. This uniqueness of plasma polymers results from the reaction mechanism of the polymer forming process.
The technique involves electric field bombardment of monomer molecules, thereby creating active monomer species, which then react with the surface to form a film on the substrate. As a result, the surface properties of the substrate change dramatically. By suitable selection of monomers, a substrate can either be made hydrophobic or hydrophilic. Plasma polymerization can be carried out at ambient temperature and does not require any solvents for the process, making it a clean process.
The surface of carbon black is known to consist of graphitic planes (site I), amorphous carbon (site II), crystallite edges (site III), and slit shaped cavities (site IV). The conduction electrons associated with the graphitic structure play an important role in the amount of energy associated with these sites. Recently, Schroder et al. [1] quantified the different energies at these sites on the surface of carbon black by analyzing adsorption isotherms of various molecules. According to their analysis, particularly the crystallite edges (III) and slit shaped cavities (IV) on the surface of carbon black are the sites of high concentration of π-electrons (see FIG. 2). These sites are most important with respect to rubber-filler and filler-filler interaction. The conduction electrons associated with the graphitic structure play an important role in the amount of energy associated with these sites.
Furthermore, the surface of carbon black is also covered with functional groups like carboxyl, phenol, lactones and quinonic groups (see FIG. 3). These are preferably located at the edges of the graphitic basal planes or at the crystallite edges.
When carbon black is exposed to plasma, the following processes can occur:
C—C bond breakage in the graphitic planes.
Due to breakage of these C—C bonds, radicals are generated on the graphitic planes. However, the graphitic structures are stabilized by resonance. As soon as radicals are generated, they will reform the bond and return to their stable state.
The breakage of C—O bonds and other functional groups, located at the crystallite edges. As soon as a C—O bond or another functional group located at the crystallite edges is broken, monomer active species can attach on to these sites, which is more favourable.
Successful attachment of the monomer active species only happens at the sites generated at the crystallite edges, i.e., at the sites generated by the bond breakage of the functional groups. For furnace carbon blacks, the concentration of these active sites (II-IV) varies between 5-20% on the surface, and the other 95-80% contribution is from graphitic planes. Furnace carbon blacks with higher surface area and lower particle size has more fractions of these energetic sites (sites II-IV). As the surface area decreases and particle size increases and the fraction of these sites decreases.
The extension of plasma polymerization as a surface modification technique for fillers like carbon black and silica for application in rubber evolved quite recently. Nah et al. [2] reported plasma polymerization on silica and its effect on rubber properties. Akovali et al [3]. and Tricas et al [4,5] reported the modification of carbon black by plasma polymerization. The monomers used for the process were acrylic acid, styrene and butadiene. Their findings led to the conclusion that carbon black was modified successfully, with the coating covering all sites on the surface of carbon black. Kang et al, [6] also reported on the modification of carbon black by plasma polymerization and concluded that it is possible to manipulate the surface properties.
However, it was found that not all types of carbon black can be successfully subjected to plasma polymerization, and that most carbon blacks are coated only with very low amounts of plasma polymer compared to silica, leading to insufficient changes of their surface properties (see, for example, Mathew et al. [7]). Other problems observed were low uniformity of the plasma polymer coated particles, long treatment times and poor reproducibility for various types of carbon black.